Method, communication system and communication unit for synchronisation for multi-rate communication

ABSTRACT

A method, communication system and communication unit for synchronisation for multi-rate communication by transmitting a signal (FIG.  4 A) having a synchronisation portion at a first, predetermined chip rate and containing an indication of chip rate used for a further portion; receiving the transmitted signal, recovering the indication from the synchronisation portion at the first, predetermined chip rate, and recovering information in the further portion at the chip rate indicated by the indication. This provides improved efficiency in supporting multi-chip rates.

FIELD OF THE INVENTION

This invention relates to digital communication systems, andparticularly to synchronisation in digital communication systems such aswireless cellular communication systems. The invention finds particularapplication in modern digital wireless communication systems such asUniversal Mobile Telecommunication Systems (UMTS).

BACKGROUND OF THE INVENTION

It is known that synchronisation is an essential procedure in a moderndigital communication system. It is the procedure used by a remote unit(often referred to as User Equipment, UE, in UMTS or Customer PremisesEquipment, CPE) to identify valid transmissions from infrastructureequipment (often referred to as Node Bs in UMTS) and align the remotefrequency reference and timing to that used by the infrastructure.

UMTS Terrestrial Radio Access (UTRA) Time Division Duplex (TDD) andFrequency Division Duplex (FDD) modes both provide a synchronisationchannel (SCH) that is used by the UE to search for valid signals andperform the synchronisation procedure. The SCH transmission consists ofone real valued Primary Synchronisation Code (PSC) and three complexSecondary Synchronisation Codes (SSC), all of length 256 chips The PSCis common for all Node Bs, but the SSCs are Node B specific. The PSC andSSC are transmitted simultaneously from a given Node B at a specificfixed time offset (t_(offset)) from the start of time slot 0. The timeoffset is included to prevent the possible capture effect that wouldotherwise occur as a consequence of all Node Bs transmitting the commonprimary code at the same time.

The UE uses the PSC to search for and identify transmissions from NodeBs. The PSC is also used as a reference from which the UE is able togenerate a correction that can be used to correct the frequency of theUE's reference oscillator. The SSC is included to signal the additionalinformation required by the UE in order to achieve the full time-alignedsynchronization and also to begin to demodulate system informationbroadcast on the Broadcast Channel (BCH) which is carried by the PrimaryCommon Control Physical Channel P-CCPCH.

For single chip-rate systems where the chip rate used by the Node B andthe UE is predetermined by the system design, the synchronizationprocedure briefly outlined above is sufficiently complete.

However, considering a network where multi-chip rates are supported, inan initial start-up condition, the UE will not be aware of the chip ratethat is available; therefore, the receiver in the UE is unable to selectthe correct chip-rate.

In some known systems such as those using fixed line modems, theavailable bandwidth is negotiated in the initial data transfers betweensender and receiver. This is done at a predetermined fixed rate, usuallydetermined by the system design or backwards compatibility with earlyimplementations.

Other possible schemes might transmit the whole timeslot in which SCHbursts are transmitted at the lower chip-rate (note that for a UMTS TDDsystem, the SCH is transmitted in every radio frame).

However, the above known fixed initial rate negotiation scheme and theother possible schemes have the disadvantage that they are inefficient.

A need therefore exists for a synchronisation scheme for multi-ratecommunication wherein the abovementioned disadvantage may be alleviated.

STATEMENT OF INVENTION

In accordance with a first aspect of the present invention there isprovided a method, for synchronisation in a multi-rate communicationsystem, the method comprising:

-   -   receiving a signal having a synchronisation portion at a first,        predetermined chip rate and containing an indication of chip        rate used for a further portion; and    -   recovering the indication from the synchronisation portion at        the first, predetermined chip rate; and    -   recovering information in the further portion at the chip rate        indicated by the indication.

In accordance with a second aspect of the present invention there isprovided a method, for synchronisation in a multi-rate communicationsystem, the method comprising:

-   -   transmitting a signal having a synchronisation portion at a        first, predetermined chip rate and containing an indication of        chip rate used for a further portion,    -   whereby the indication may be recovered from the synchronisation        portion at the first, predetermined chip rate; and information        in the further portion may be recovered at the chip rate        indicated by the indication.

In accordance with a third aspect of the present invention there isprovided a multi-rate communication system comprising:

-   -   a transmitter having means for transmitting a signal having a        synchronisation portion at a first, predetermined chip rate and        containing an indication of chip rate used for a further        portion;    -   a receiver having    -   means for receiving the transmitted signal,    -   means for recovering the indication from the synchronisation        portion at the first, predetermined chip rate, and    -   means for recovering information in the further portion at the        chip rate indicated by the indication.

In accordance with a fourth aspect of the present invention there isprovided a communication unit, for use in a multi-rate communicationsystem, the communication unit comprising:

-   -   means for receiving a signal having a synchronisation portion at        a first, predetermined chip rate and containing an indication of        chip rate used for a further portion;    -   means for recovering the indication from the synchronisation        portion at the first, predetermined chip rate; and    -   means for recovering information in the further portion at the        chip rate indicated by the indication.

In accordance with a fifth aspect of the present invention there isprovided a communication unit, for use in a multi-rate communicationsystem, the communication unit comprising:

-   -   means for transmitting a signal having a synchronisation portion        at a first, predetermined chip rate and containing an indication        of chip rate used for a further portion,    -   whereby the indication may be recovered from the synchronisation        portion at the first, predetermined chip rate; and information        in the further portion may be recovered at the chip rate        indicated by the indication.

BRIEF DESCRIPTION OF THE DRAWINGS

One method, communication unit and communication system forsynchronisation for multi-rate communication incorporating the presentinvention will now be described, by way of example only, with referenceto the accompanying drawings, in which:

FIG. 1 shows a block diagram of a wireless communication system that canbe adapted to support the various inventive concepts of a preferredembodiment of the present invention;

FIG. 2 shows a block diagram of a wireless communication unit that canbe adapted to support the various inventive concepts of a preferredembodiment of the present invention;

FIG. 3 shows a block schematic diagram illustrating SCH transmission andreception in a single chip rate system incorporating the invention; and

FIG. 4 shows a block schematic diagram illustrating SCH transmission andreception in a multi chip-rate system incorporating the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to FIG. 3, a multi-rate cellular-based wireless telephonecommunication system 100 is shown in outline, in accordance with apreferred embodiment of the invention. Preferably, the cellular-basedtelephone communication system 100 is compliant with, and containsnetwork elements capable of operating over, a UMTS air-interface. Inparticular, the invention relates to the Third Generation PartnershipProject (3GPP) specification for wide-band code-division multiple access(WCDMA) standard relating to the UTRAN Radio Interface (described in the3G TS 25.xxx series of specifications).

A plurality of subscriber terminals (or user equipment (UE) in UMTSnomenclature) 112, 114, 116 communicate over radio links 118, 119, 120with a plurality of base transceiver stations, referred to under UMTSterminology as Node-Bs, 122, 124, 126, 128, 130, 132. The systemcomprises many other UEs and Node Bs, which for clarity purposes are notshown.

The wireless communication system, sometimes referred to as a NetworkOperator's Network Domain, is connected to an external network 134, forexample the Internet. The Network Operator's Network Domain includes:

-   -   (i) A core network, namely at least one Gateway GPRS Support        Node (GGSN) 144 and or at least one Serving GPRS Support Nodes        (SGSN); and    -   (ii) An access network, namely:        -   (ai) a GPRS (or UMTS) Radio network controller (RNC)            136-140; or        -   (aii) Base Site Controller (BSC) in a GSM system and/or        -   (bi) a GPRS (or UMTS) Node B 122-132; or        -   (bii) a Base Transceiver Station (BTS) in a GSM system.

The GGSN/SGSN 144 is responsible for GPRS (or UMTS) interfacing with aPublic Switched Data Network (PSDN) such as the Internet 134 or a PublicSwitched Telephone Network (PSTN) 134. A SGSN 144 performs a routing andtunnelling function for traffic within say, a GPRS core network, whilsta GGSN 144 links to external packet networks, in this case onesaccessing the GPRS mode of the system.

The Node-Bs 122-132 are connected to external networks, through basestation controllers, referred to under UMTS terminology as Radio NetworkController stations (RNC), including the RNCs 136, 138, 140 and mobileswitching centres (MSCs), such as MSC 142 (the others are, for claritypurposes, not shown) and SGSN 144 (the others are, for clarity purposes,not shown).

Each Node-B 122-132 contains one or more transceiver units andcommunicates with the rest of the cell-based system infrastructure viaan Iub interface, as defined in the UMTS specification.

Each RNC 136-140 may control one or more Node-Bs 122-132. Each MSC 142provides a gateway to the external network 134. The Operations andManagement Centre (OMC) 146 is operably connected to RNCs 136-140 andNode-Bs 122-132 (shown only with respect to Node-B 126 for clarity). TheOMC 146 administers and manages sections of the cellular telephonecommunication system 100, as is understood by those skilled in the art.

In the preferred embodiment of the invention, at least one UE 312-316and at least one Node-B 322-332 have been adapted, to offer, and providefor, transmission, reception and processing of multi-rate high-speedsignals generated in accordance with the approach discussed in detailbelow.

More particularly, in this embodiment the above elements have beenadapted to implement the present invention in both transmitting andreceiving modes of operation, such that in this embodiment the inventionmay be applied to both down-link and up-link transmissions.

It is also within the contemplation of the invention that suchadaptation of the physical layer (air-interface) elements mayalternatively be controlled, implemented in full or implemented in partby adapting any other suitable part of the communication system 100. Forexample, equivalent parts in other types of systems may, in somecircumstances, be adapted to provide some or all of the digitalfiltering implementation provided in this embodiment.

Further, in the case of other network infrastructures, implementation ofthe processing operations may be performed at any appropriate node suchas any other appropriate type of base station, base station controller,etc.

Alternatively the aforementioned digital filtering operations may becarried out by various components distributed at different locations orentities within any suitable network or system.

Although the preferred embodiment of the invention is described withreference to a wireless communication system employing a UMTSair-interface, it is within the contemplation of the invention that theinventive concepts described herein can be applied to anymulti-bandwidth/multi-data rate communication system—fixed or wireless.

Referring now to FIG. 2, a block diagram of a communication unit 200,for example user equipment (UE) 112, adapted to support the inventiveconcepts of the preferred embodiments of the present invention, isshown. However, it is within the contemplation of the invention that asimilar block diagram would apply to a Node B element, say Node B 122.Therefore, in the following description FIG. 2 is described such that italso encompasses an implementation of a Node B baseband processingcircuit, in broad principle, as would be appreciated by a person skilledin the art.

The UE 112 contains an antenna 202 preferably coupled to a duplex filteror circulator or switch 204 that provides isolation between receive andtransmit chains within UE 112.

The receiver chain includes scanning receiver front-end circuitry 206(effectively providing reception, filtering and intermediate or basebandfrequency conversion). The scanning front-end circuit 206 scans signaltransmissions from its associated Node B. The scanning front-end circuit206 is serially coupled to a signal processing function (processor,generally realised by a DSP) 208. The final receiver circuits are abaseband back-end circuit 209 operably coupled to a display unit 210, ifthe communication unit is a subscriber unit.

Alternatively, if the communication unit is a Node B, the final receivercircuits are a baseband back-end circuit 209 operably coupled to aninterface port 210, in order to forward the demodulated received signalto, say, a PC or a RNC.

In accordance with a preferred embodiment of the invention, the receiverchain, in particular the signal processing function 208, coupled to thescanning baseband back-end circuit 209, has been adapted for a receivingcommunication unit to receive and process multiple, high-speed signalsof varying bandwidths.

A controller 214 is operably coupled to the scanning front-end circuitry206 so that the receiver can calculate receive bit-error-rate (BER) orframe-error-rate (FER) or similar link-quality measurement data fromrecovered information via a received signal strength indication (RSSI)212 function. The RSSI 212 function is operably coupled to the scanningfront-end circuit 206. A memory device 216 in the controller 214 storesa wide array of UE-specific data, such as decoding/encoding functions,timing details, neighbour and serving cell information relating totiming, channels, power control and the like, as well as link qualitymeasurement information to enable an optimal communication link to beselected.

A timer 218 is operably coupled to the controller 214 to control thetiming of operations, namely the transmission or reception oftime-dependent signals, within the UE 112.

In the context of the preferred embodiment of the present invention,timer 218 is used to synchronize the timing of the receivingcommunication unit 200 to be able to switch between two or more filterconfigurations, as will be described below, as well as to co-ordinateappropriate clocking of signals throughout the receiver.

For completeness, in broad terms, the transmit chain of thecommunication unit (either a UE or Node B) essentially includes an inputdevice 220, coupled in series through the processor 208,transmitter/modulation circuitry 222 and a power amplifier 224. Theprocessor 208, transmitter/modulation circuitry 222 and the poweramplifier 224 are operationally responsive to the controller 214, withan output from the power amplifier coupled to the duplex filter orcirculator 204, as known in the art.

The signal processor function 208 in the transmit chain may beimplemented as distinct from the processor in the receive chain.Alternatively, a single processor 208 may be used to implementprocessing of both transmit and receive signals, as shown in FIG. 2.

Of course, it will be understood that the various components within thecommunication unit 200 can be realised in discrete or integratedcomponent form, with an ultimate structure therefore being merely anarbitrary selection.

More generally, the digital filtering algorithms associated with thepreferred embodiment of the present invention may be implemented in arespective communication unit in any suitable manner. For example, newapparatus may be added to a conventional communication unit (for exampleUE 112, or Node B 122), or alternatively existing parts of aconventional communication unit may be adapted, for example byreprogramming one or more processors therein. As such the requiredadaptation may be implemented in the form of processor-implementableinstructions stored on a storage medium or data carrier, such as afloppy disk, hard disk, PROM, RAM or any combination of these or otherstorage multimedia.

This invention, at least in a preferred form, implements a scheme wherethe SCH channel in the UTRA air-interface is transmitted at the lowestchip-rate supported by the system design. Note that only the SCH channelis always transmitted at the lower chip rate.

As the SCH is transmitted at the lower chip rate, the receiving UE willby default, select the receiver bandwidth appropriate to this lowerchip-rate. In this configuration, the UE will be able to recover theSCH, irrespective of the chip rate used at the transmitting Node B.

The modulation of data onto the secondary SCH defined by the UTRAstandard does not use all of the degrees of freedom available in themodulation scheme. Therefore, the mapping of the synchronisationspecific data on to the SSC can be expanded to allow the additionalsignalling of the transmitting Node B chip rate to be added (see GBpatent application no. 0122109.2, filed on 13 Sep. 2001 by the sameapplicant as the present application and entitled “ENCODER AND METHODFOR EFFICIENT SYNCHRONISATION CHANNEL ENCODING IN UTRA TDD MODE”, thecontent of which is hereby incorporated herein by reference).

A simplified diagram of the single chip-rate implementation of apreferred embodiment of the invention is shown in FIG. 3.

In this example, the SCH is treated identically to the rest of the databurst. That is, the SCH is processed by the same transmit and receivefilters as the physical channels used to transport the informationhaving the same chip rate.

Thus, as shown in FIG. 3A, in the transmit path of the transmitting NodeB a combiner 310 combines SCH information 320 with the appropriate databurst construct 330. The resultant data burst containing the SCHinformation is filtered in the digital low-pass transmit filter 340(which may, for example, be of the ‘root-raised cosine’ type). Theanalogue section 350 of the transmitter is set to the bandwidth(narrowest) appropriate for the lowest chip rate, and the data burst ispassed to the antenna for transmission.

Correspondingly, as shown in FIG. 3B, in the receive path of thereceiving UE the analogue section 360 of the receiver is set to thebandwidth (narrowest) appropriate for the lowest chip rate, and performsinitial filtering of the data burst received at the antenna. The outputof the analogue section 360 is then filtered in the digital low-passreceive filter 370 (which may, like the digital transmit filter 340, beof the ‘root-raised cosine’ type). The output of the digital low-passreceive filter 370 is processed to recover the SCH information and (aswill be explained in greater detail below) to decode the system chiprate information therefrom (as depicted at 380). Since (in this singlechip rate case) the decoded system chip rate information does notindicate that the system chip rate is different than the chip rate usedfor the SCH information (i.e., it indicates that a single chip rate isused), the receive path digital filters remain configured for thesingle, lowest chip rate for subsequent processing of the data burst (asindicated at 390) and transport channel information as for the SCHinformation.

Referring now also FIG. 4, in the case where a different chip-rate isavailable for the physical channel that is used to transport data, it isnecessary to provide different filters (or to differently configure thefilter(s)) for the SCH channel and the physical channels used totransport the data. Such different filters, or re-configuration of thesame filter(s), may be implemented as in GB patent application no.018414.2, filed on 30 Jul. 2001 by the same applicant as the presentapplication and entitled “DIGITAL FILTER FOR MULTI-RATE COMMUNICATION”,the content of which is hereby incorporated herein by reference.

Suppose the chip rate in a multi chip-rate system is given byf_(c)=nf_(b); n=1, . . . , Nwhere f_(b) is the base chip rate and N is the number of available chiprates in the multi-chip rate system. When a UE is initialised it knows apriori that the chip-rate being used for the SCH is f_(b), but it doesnot know the system chip rate being used, f_(c). In the Node Btransmitter, it is necessary to pass the SCH physical channel through afilter (typically a digital filter) optimised for f_(b). The physicalchannels transporting the data are filtered with a (digital) filteroptimised for f_(c). In the analogue section of the Node B transmitter,the filter bandwidth is always equal to f_(c).

In the receive section of the user equipment, the receiver bandwidth isset to f_(b) in both the analogue section and digital sections. In thisconfiguration, the physical channels with chip-rate f_(c) may suffersevere inter-symbol interference when f_(c)≠f_(b). However, the SCHphysical channel is received with minimal degradation. It is necessaryto use a bandwidth of f_(b) in the analogue filter and the digitalfilter in order to apply maximum attenuation to potentially high-poweradjacent channel interferers.

With a UE is in this configuration, it is possible to demodulate the SCHchannel and decode the data transported by the SSC to determine f_(c).When initial synchronisation has been achieved, the analogue and digitalfilters are set to f_(c).

FIG. 4 shows the receiver/transmitter implementation of this multi-chiprate scheme.

Thus, as shown in FIG. 4A, in the transmit path of the transmitting NodeB a combiner 310 combines SCH information 320 (filtered by a digitallow-pass filter 325 set to the low chip rate f_(b) so as to ensure thatthe SCH information can be recovered in the receiver by filtering atthis chip rate) with the appropriate data burst construct 330. The SCHinformation is encoded with the desired higher system chip rate f_(c),as explained in detail in the above-mentioned GB patent application no.018414.2. The resultant data burst containing the SCH information isfiltered in the digital low-pass transmit filter 340 (now set for thedesired high chip rate f_(c)). The analogue section 350 of thetransmitter is set to a bandwidth (wider than in the case of FIG. 3A)appropriate for the higher chip rate, and the data burst is passed tothe antenna for transmission.

Correspondingly, in the receive path of the receiving UE, in a firststate, as shown in FIG. 4B, the analogue section 360 of the receiver isset to the bandwidth (narrowest) appropriate for the lowest chip rate,and performs initial filtering of the data burst received at theantenna. The output of the analogue section 360 is then filtered in thedigital low-pass receive filter 370. The output of the digital low-passreceive filter 370 is processed to recover the SCH information anddecode the system chip rate information therefrom. It will beappreciated that this initial stage of receive path processing issimilar to that shown and described above in relation to the singlechip-rate case shown in FIG. 3A. As will be explained further below, atthis stage (since the indicated system chip rate f_(c) is higher thanthe lowest chip rate f_(b) used for the SCH information) data burstprocessing is disabled (as indicated at 395).

In this multi chip-rate case, the system chip rate information decodedfrom the SCH information indicates the higher chip rate used fortransport channel information. Since this indicated system chip ratef_(c) is higher than the low chip rate f_(b) used for the SCHinformation, the receive path is then configured into a second state, asshown in FIG. 4C, in which the analogue section 360 and the digital lowpass receive filter 370 are set to badwidths appropriate for the higherchip rate f_(c).

In this second state, in the receive path of the receiving UE theanalogue section 360 of the receiver performs (now at the higherbandwidth appropriate for the higher chip rate f_(c)) filtering of thesignals received at the antenna. The output of the analogue section 360is then filtered (now at the higher bandwidth appropriate for the higherchip rate f_(c)) in the digital low-pass receive filter 370. The outputof the digital low-pass receive filter 370 is then processed (i) torecover the data burst information (now enabled, as depicted at 390) andtransport channel information at the higher chip rate, and (ii) tofurther process (after filtering by a digital low-pass filter 385 set tothe low chip rate f_(b) so as to ensure that the SCH information can berecovered in the receiver by filtering at this chip rate) the SCHinformation (as depicted at 380).

It will be understood that the method, communication unit andcommunication system for synchronisation for multi-rate communicationdescribed above provides improved efficiency in supporting multi-chiprates.

1. A method for synchronisation in a multi-rate communication system,the method comprising: receiving a signal having a synchronisationportion at a first, predetermined chip rate and containing an indicationof chip rate used for a further portion; and recovering the indicationfrom the synchronisation portion at the first, predetermined chip rate;and recovering information in the further portion at the chip rateindicated by the indication.
 2. The method of claim 1, wherein the stepof recovering the indication comprises processing the synchronisationportion by filter means set at a bandpass appropriate for the first,predetermined chip rate, and the step of recovering information in thefurther portion comprises processing the further portion by filter meansset at a bandpass appropriate for the indicated chip rate.
 3. The methodof claim 2, wherein the filter means processing the synchronisationportion and the filter means processing the further portion comprisecommon, re-configurable filter means.
 4. The method of claim 1, whereinthe first, predetermined chip rate is lower than the indicated chiprate.
 5. The method of claim 1, wherein the signal comprises a databurst and the synchronisation portion comprises a synchronisationchannel signal.
 6. The method of claim 1, wherein the system is awireless communication system.
 7. The method of claim 6, wherein thesystem is a UMTS system.
 8. A method for synchronisation in a multi-ratecommunication system, the method comprising: transmitting a signalhaving a synchronisation portion at a first, predetermined chip rate andcontaining an indication of chip rate used for a further portion,whereby the indication may be recovered from the synchronisation portionat the first, predetermined chip rate; and information in the furtherportion may be recovered at the chip rate indicated by the indication.9. The method of claim 8, wherein the first, predetermined chip rate islower than the indicated chip rate.
 10. The method of claim 8, whereinthe signal comprises a data burst and the synchronisation portioncomprises a synchronisation channel signal.
 11. The method of claim 8,wherein the system is a wireless communication system.
 12. The method ofclaim 11, wherein the system is a UMTS system.
 13. A multi-ratecommunication system comprising: a transmitter having means fortransmitting a signal having a synchronisation portion at a first,predetermined chip rate and containing an indication of chip rate usedfor a further portion; a receiver having means for receiving thetransmitted signal, means for recovering the indication from thesynchronisation portion at the first, predetermined chip rate, and meansfor recovering information in the further portion at the chip rateindicated by the indication.
 14. The system of claim 13, wherein themeans for recovering the indication comprises filter means set at abandpass appropriate for the first, predetermined chip rate, and themeans for recovering information in the further portion comprises filtermeans set at a bandpass appropriate for the indicated chip rate.
 15. Thesystem of claim 14, wherein the filter means set at a bandpassappropriate for the first, predetermined chip rate and the filter meansset at a bandpass appropriate for the indicated chip rate comprisecommon, re-configurable filter means.
 16. The system of claim 13,wherein the first, predetermined chip rate is lower than the indicatedchip rate.
 17. The system of claim 13, wherein the signal comprises adata burst and the synchronisation portion comprises a synchronisationchannel signal.
 18. The system of any claim 13, wherein the system is awireless communication system.
 19. The system of claim 18, wherein thesystem is a UMTS system.
 20. A communication unit for use in amulti-rate communication system, the communication unit comprising:means for receiving a signal having a synchronisation portion at afirst, predetermined chip rate and containing an indication of chip rateused for a further portion; means for recovering the indication from thesynchronisation portion at the first, predetermined chip rate; and meansfor recovering information in the further portion at the chip rateindicated by the indication.
 21. The communication unit of claim 20,wherein the means for recovering the indication comprises filter meansset at a bandpass appropriate for the first, predetermined chip rate,and the means for recovering information in the further portioncomprises filter means set at a bandpass appropriate for the indicatedchip rate.
 22. The communication unit of claim 21, wherein the filtermeans set at a bandpass appropriate for the first, predetermined chiprate and the filter means set at a bandpass appropriate for theindicated chip rate comprise common, re-configurable filter means. 23.The communication unit of claim 20, wherein the first, predeterminedchip rate is lower than the indicated chip rate.
 24. The communicationunit of claim 20, wherein the signal comprises a data burst and thesynchronisation portion comprises a synchronisation channel signal. 25.The communication unit of claim 20, wherein the system is a wirelesscommunication system.
 26. The communication unit of claim 25, whereinthe system is a UMTS system.
 27. A communication unit for use in amulti-rate communication system, the communication unit comprising:means for transmitting a signal having a synchronisation portion at afirst, predetermined chip rate and containing an indication of chip rateused for a further portion, whereby the indication may be recovered fromthe synchronisation portion at the first, predetermined chip rate; andinformation in the further portion may be recovered at the chip rateindicated by the indication.
 28. The communication unit of claim 27,wherein the first, predetermined chip rate is lower than the indicatedchip rate.
 29. The communication unit of claim 27, wherein the signalcomprises a data burst and the synchronisation portion comprises asynchronisation channel signal.
 30. The communication unit of claim 27,wherein the system is a wireless communication system.
 31. Thecommunication unit of claim 30, wherein the system is a UMTS system. 32.The communication unit of claim 20, wherein the communication unit isone of: a user equipment, Node B.
 33. A computer program elementcomprising computer program means for performing the method forsynchronisation in a multi-rate communication system encoding functionsas claimed in claim 1.